1. Field of the Invention
The present invention relates generally to hydrophilic ultraviolet light absorbing monomers. More particularly, this invention relates to ultraviolet light absorbing monomers polymerizable as co-monomers with other polymerizable monomers and co-polymers. In one of its more particular aspects, this invention relates to an ultraviolet light absorbing monomer polymerizable as a co-monomer with other suitable hydrophilic monomers into optically transparent, high refractive index hydrogels which are especially useful in the fabrication of intraocular lenses and contact lenses.
2. Description of Related Art
Optical devices in the form of intraocular lenses and contact lenses have been commercially available for several decades. For contact lenses, the primary indication for use has been as an adjunct for improving the wearer's visual acuity. This is accomplished by adding or subtracting small amounts of diopter power to the surface of the cornea. In addition, the contact lens also may have correction for astigmatism. A contact lens should be stable at temperatures at, or below, body temperature in an aqueous environment, non-toxic and not contain leachable compounds.
With intraocular lenses, the primary indication for use has been for the replacement of the natural crystalline lens of humans and other mammals that were lost to injury and/or cataract formation. The natural lens is generally a biconvex lens, from 6 to 13 mm in width, that has considerable optical power, nearly 20 diopters. Therefore, compared to a contact lens, replacing a damaged natural lens requires the use of a substantially larger, thicker, intraocular implant lens. Like the contact lens, an intraocular lens should be stable at body temperature in an aqueous environment, non-toxic and not contain leachable compounds.
Early on, the materials of choice for forming intraocular and contact lenses were the acrylates and methacrylates, particularly polymethylmethacrylate. These materials form rigid, glass-like lenses that are easily shaped to the desired optical correction. These compounds are successful as contact lenses and are generally known as the "hard" contact lenses.
These rigid, glassy polymers necessarily need to have diameters in the range of 6-13 mm to function as intraocluar lenses. Because these lenses are rigid, this limitation requires incisions into the eye of a correspondingly equal width. However, early surgical techniques to remove damaged lenses used large incisions, so the large size was not perceived as that great of a drawback. Such large incisions entail numerous complications and have protracted healing times. Advancements in the surgical technique for removing native lenses now provide for using smaller and smaller incisions, down in the 2-3 mm range. Consequently, the search was on to find suitable materials for use as an intraocular lens that could be inserted through smaller incisions.
To alleviate the drawbacks of using polyacrylates, such as polymethylmethacrylate, various hydrogels and elastomeric silicones have been developed that are rollable, foldable or deformable, yet resilient. When folded or deformed, the lens may be inserted into the eye through incisions as small as 2-3 mm. The resiliency of these materials provides for these lenses to re-assume their original biconvex optical shape after insertion. The materials used in these soft lenses have proven to provide optically clear lenses with sufficient indices of refraction, yet are strong or resilient enough to withstand the folding, deformation or rolling processes needed to achieve the smaller incision sizes. The folding, deforming or rolling capabilities of these substances, providing for smaller incisions, is a substantial improvement for the patient in terms of reduced trauma to the eye, improved post surgical healing and reduction in complications.
Softness and resilience is not the only improvement that has been sought. Another improvement being sought is ultraviolet (UV) light protection. As research into UV light exposure progresses, our understanding of the numerous deleterious effects of UV light exposure is growing. More and more products are being developed each year to decrease or prevent exposure to the harmful effects of UV light. UV light absorption for contact and intraocular lenses is at least as important as UV absorption for skin found in sun screens. What amount of UV light protection a native lens provides is lost when it is removed, increasing the risk to the retina from deleterious exposure to UV light if that protection is not restored. UV light protection for the eye may be enhanced by providing UV absorbers in contact lenses or in intraocular lens implants.
The use of UV light absorbers in hydrogels for use in contact and intraocular lenses poses other problems, as well. Optical hydrogels suitable for use in contact or intraocular lenses need clarity, good optical power, stability and resilience. Because of the long term use of contact and intraocular lenses, especially for intraocular lenses, the UV light absorbing compound should stay put within the copolymer. If the UV light absorber leaches out, there is the risk to the surrounding tissue from the chemical exposure. There is also the increasing risk from UV exposure as the UV light absorption capability diminishes over time.
UV light absorbers for use in hydrogels should be polymerizable as a comonomer in the hydrogel. Benzophenone based UV absorbers are polymerizable, but the resultant polymers may not be thermally stable, particularly when hydrated. The UV absorbing portion cleaves and leaches out of the polymer.
In addition, these UV absorbers are hydrophobic and not very soluble with hydrophilic hydrogel comonomers and copolymers. Even though these compounds are somewhat soluble with hydrophilic comonomers and copolymers, when hydrated within a hydrogel, they tend to coalesce from microphase separation. This coalescence clouds the material rendering it undesirable for use as a lens.
Another general class of UV light absorber is the class of phenylbenzotriazoles, such as 2-(2'-hydroxy-5'-methacryloxyethylphenyl)-2H-benzotriazole and its derivatives. These derivatives are polymerizable and stable against hydrolysis, and like the phenol based UV light absorbers, these compounds tend to be hydrophobic. The hydrophobic characteristic leads to microphase separation and clouding.
The hydrophobicity of the UV light absorber also decreases the amount of water absorption into the hydrogel. The decreased water absorption creates a harder, less resilient hydrogel material. To counter these drawbacks, the amount of UV light absorber is kept to a minimum to make a resilient, optically clear hydrogel. As a consequence, current use of hydrophobic UV light absorbers in optically clear material do not produce hydrogels with substantial UV light absorbing characteristics. In today's regulatory environment, substantial UV light absorption is at least 90% absorption of light at or below 372 nm wavelength.
It is, therefore, an object of the present invention to provide stable, polymerizable UV light absorbing monomers having increased hydrophilicity.
Another object of the present invention is to provide stable increased hydrophilicity UV light absorbing hydrogels having the properties of optical transparency and resiliency while substantially absorbing UV light.
Other objects and advantages of the present invention will become apparent from the following disclosure and description.